Characterization of integration host factor (IHF) binding upstream of the cysteine-rich protein operon (omcAB) promoter of Chlamydia trachomatis LGV serovar L2.

Chlamydiae are bacterial parasites that carry out a distinct developmental cycle within host cells; however, the mechanisms by which these organisms regulate stage-specific gene expression are not known. We identified a DNA element located between nucleotide (nt) -135 and -90 upstream from the transcription start point of the late stage-specific CRP operon (omcAB) of Chlamydia trachomatis, to which a protein in extracts of chlamydiae harvested at 23 h after infection binds. A recombinant protein of C. trachomatis open reading frame (ORF) CT267, which is homologous to bacterial integration host factor (IHF) and the heat-unstable nucleoid protein (HU), bound to the same element and produced the same DNase I footprint as the protein in chlamydial extracts. Recombinant ORF CT267 protein bound with high affinity to the DNA element and induced a sharp bend in a DNA fragment containing the binding site, suggesting that ORF CT267 encodes a protein with IHF-like activity, and recombinant protein had a positive effect on in vitro transcription of the CRP operon. IHF-binding activity and IHF protein were detected in extracts of C. trachomatis during the early to intermediate phases of the late stage of the developmental cycle (between 17 and 30 h after infection), but were absent in the extreme late phase of the cycle and in the infectious form of chlamydiae. The presence of an IHF binding site upstream of the CRP operon and the presence of chlamydial IHF-like protein when late stage genes are transcribed suggests that the chlamydial IHF may play a role in stage-specific gene expression.